Process and apparatus for manufacture of processable polyvinyl alcohol

ABSTRACT

The invention describes a method for manufacture of plasticized homopolymeric polyvinyl alcohol, the method including the steps of introducing a polyvinyl alcohol polymer or a blend thereof having a degree of hydrolysis in the range of 93 wt % to 98 wt % into a mixing reactor; adding a processing agent, a plasticizer and a reactive stabilizer to form a reaction mixture; wherein the plasticizer is selected from the group consisting of sugar alcohols, diols, triols, polyols and mixtures thereof; wherein the reactive stabilizer is selected from the group consisting of sodium stearate, potassium oleate, sodium benzoate, calcium stearate, stearic acid, dimethyl propionic acid, and mixtures thereof;-reacting the reaction mixture in a reaction zone to form plasticized polymer; and-allowing the plasticized polymer to pass from the reaction zone.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority to European Application Patent SerialNo. 20184345.5, filed Jul. 6, 2020, the entire disclosure of which ishereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a process for manufacture of polyhydricpolymers, particularly polyvinyl alcohol. The invention also relates toapparatus for use in carrying out the method. The invention furtherrelates particularly but not exclusively to a method of manufacture ofpolyvinyl alcohol in a suitable physical form for processing into filmor other extruded products. The invention further relates to a novelprocessable polyvinyl alcohol composition which may be made using theprocess.

BACKGROUND

Polyvinyl alcohol is commonly made by hydrolysis of polyvinyl acetate.The degree of hydrolysis affects the properties of the polymer.Polyvinyl alcohol having a low degree (LD) of hydrolysis, below 84%, iswidely used in industry. Vinyl acetate copolymers, for example, withethylene acetate have been used to make vinyl alcohol co-polymers whichare easier to process. However, these co-polymers lack the advantageousphysical properties of homopolymer polyvinyl alcohol, particularlyhighly hydrolysed polyvinyl alcohol homopolymer. The present inventionrelates particularly to polyvinyl alcohol made by hydrolysis ofhomopolymeric polyvinyl acetate.

Highly hydrolysed polyvinyl acetate, that is with a degree of hydrolysisgreater than 93, for example, 98% or higher, is a polymer whichessentially comprises homo-polyvinyl alcohol. This polymer, similar tomany carbohydrates, decomposes before its melting point of about 250° C.is reached. This makes melt processing difficult and for this reason thehighly hydrolysed polyvinyl alcohol polymer has been processed as anaqueous solution. Partially hydrolysed polyvinyl acetate is readily meltprocessed. For example, 80% hydrolysed polyvinyl acetate, can be readilyextruded or converted into film by blow molding.

The significant difference between highly hydrolysed (high degree ofhydrolysis, HD) and partially hydrolysed (low degree of hydrolysis, LD)polyvinyl alcohols is the extent and quality of the crystalline orderdue to the differences in the chain structures. Polyvinyl alcohols withless than 2% non-hydrolysed acetate groups can readily crystallise toform strongly hydrogen bonded crystalline domains. These crystallinedomains have a structure which is essentially the same as found inpolyethylene. The reason for this may be attributed to the small size ofthe hydroxyl group. However, because of the hydrogen bonding, themelting point of highly hydrolysed polyvinyl alcohol is about 150° C.higher than that of polyethylene. Polyols have been used asplasticizers, but efficient manufacture of plasticised polyvinyl alcoholwith a high degree of hydrolysis has been difficult to achieve.

WO 2017/046361 discloses a method for manufacture of a plasticizedpolyvinyl alcohol having a degree of hydrolysis of 98 wt % or higher.

DETAILED DESCRIPTION

According to a first aspect of the present invention, there is provideda method for the manufacture of plasticised polyvinyl alcohol polymer,the method comprising the steps of:

introducing into a mixing reactor a polyvinyl alcohol polymer comprisinghomopolymeric polyvinyl alcohol or a blend thereof having a degree ofhydrolysis in the range of 93 wt % to 98 wt % or more;

wherein the mixing reactor comprises a blending chamber having a primaryinlet, a primary outlet and at least two inter-engaging componentsextending between the primary inlet and primary outlet, the componentsbeing arranged to apply a shearing force to the polymer while thepolymer is conveyed by the components from the inlet through a reactionzone to the outlet;

one or more secondary inlets located downstream from the primary inletfor introducing reactants comprising a processing aid, a plasticizer anda reactive stabilizer to the chamber to form a reaction mixture;

wherein the plasticizer is selected from the group consisting of: sugaralcohols, diols, triols, polyols and mixtures thereof;

wherein the reactive stabilizer is selected from the group consistingof:

sodium stearate, potassium oleate, sodium benzoate, calcium stearate,stearic acid, dimethyl propionic acid, and mixtures thereof;

wherein the blending chamber comprises a plurality of heated regionsarranged so that the mixture is subjected to a temperature profilewhereby the temperature increases from the inlet to the outlet;

a secondary outlet located between the reaction zone and primary outletarranged to allow removal of processing aid from the chamber;

reacting the processing agent, plasticizer and polymer in the reactionzone to form plasticised polymer; and

allowing the plasticised polymer to pass from the primary outlet.

Use of a reactive mixing apparatus, typically an extruder in accordancewith this invention allows the processing aid and plasticizer to bereacted with the polyvinyl alcohol or blend thereof, withoutdecomposition of the polymer followed by removal of all or most of theprocessing aid from the secondary outlet to give plasticised polyvinylalcohol or a blend thereof.

Use of a reactive stabilizer may result in an advantageous reduction inthe extent of degradation during melt processing. This allowshomopolymeric polyvinyl alcohol having a high degree of hydrolysis, forexample 93 wt % or higher to be processed to form pellets, films andfibres.

The reactive stabilizer may be used in an amount of about 0.2 wt % toabout 5 wt %, for example about 0.5 wt % to about 3 wt %, for example0.5 wt % to about 2 wt %, for example from about 0.5 wt % to about 1.5wt %, for example about 1 wt %.

The reactive stabilizers of this invention may decrease the extent ofdegradation of the polymer during processing. Homopolymeric polyvinylalcohol has been difficult to process due to degradation at the hightemperatures required. The liability of degradation has led to use ofpolyvinyl alcohol co-polymers with a consequent loss of engineeringproperties. This can be seen by UV spectral analysis of the amount ofconjugation present in the polymer. Sodium benzoate has been found to beparticularly effective.

In exemplary embodiments thin films of melt processed polyvinyl alcoholformulations of this invention remain clear and do not show a whiteningeffect after exposure to humidity. Use of an appropriate reactivestabilizer may provide films with improved visual appearance. Apreferred reactive stabilizer is sodium benzoate. In less advantageousformulations formation of an opaque, hazy or white film after a 24-hourhumidity test may indicate that phase separation has occurred.

The polyvinyl alcohol polymer may comprise polyvinyl alcohol or a blendthereof wherein the polyvinyl alcohol polymer has a degree of hydrolysisof 93 wt % to less than 98 wt %, preferably 93 wt % to 97 wt %,alternatively 93 wt % to 95 wt %.

Use of homopolymeric polyvinyl alcohol is particularly advantageous.Homopolymeric polyvinyl alcohol is manufactured by hydrolysis ofhomopolymeric polyvinyl acetate, the degree of hydrolysis being 93 wt %or more in embodiments of this invention. Polyvinyl alcohol co-polymersmade by hydrolysis of polyvinyl acetate co-polymers have inferiorproperties compared to homopolymeric polyvinyl alcohol. Homopolymericpolyvinyl alcohol may exhibit the following advantageous properties.

Polyvinyl alcohol polymers of this invention may have excellentfilm-forming, emulsifying and adhesive properties. The polymers exhibitexcellent barrier properties including resistance to oil, grease andsolvents. The polymers may also have high tensile strength andflexibility, as well as high oxygen and aroma barrier properties.

The polyvinyl alcohol may be manufactured by hydrolysis of homopolymericpolyvinyl acetate, wherein the extent of hydrolysis is in the range from93 wt % up to 98 wt %, for example 93 wt % to less than 98 wt %, forexample 93 wt % to 97 wt %, for example 93 wt % to 95 wt %.

A blend of two or more polyvinyl alcohol polymers may be employed, forexample a blend of two polyvinyl alcohol polymers with a relatively highmolecular weight and a relatively low molecular weight respectively.

A blend of polyvinyl alcohols with different hydrolysis levels can becombined. Blending different polyvinyl grades together enables theproperties of the resultant polymer to be enhanced, for example,viscosity, solubility and melt strength.

A blend of two polyvinyl alcohol polymers having the same degree ofhydrolysis but with different viscosities may be employed. For example,one polymer may have a viscosity of 5 cp and the other may have aviscosity of 28 cp in order to provide a polymer with a specific desiredviscosity. The viscosity may be adjusted by changing the ratio of lowerviscosity polymer to higher viscosity polymer. In an embodiment, a blendwhere the ratio of higher viscosity to lower viscosity polyvinylalcohols is 80:20 wt % the resultant polymer may be more viscous than ablend with a 40:60 wt % ratio. This enables properties of the polymer tobe controlled for use in specific applications. Controlling combinationswith different degrees of hydrolysis and ratios of relative weights ofthe polymers allows consequent control of the solubility of the polymer.For example, two polymers with the same plasticizers, one with a blendof polyvinyl alcohol with a degree of hydrolysis greater than 98% mayhave a dissolution temperature of 70° C. in water, whereas the blend ofpolyvinyl alcohol with a degree of hydrolysis of 87-96% may have adissolution temperature of 40° C. in water. A blend of the samepolyvinyl alcohols in a ratio of 80:20 wt % may be soluble in water at40° C., whereas with a ratio of 60:40 wt %, the dissolution temperaturemay be 30° C. This is illustrated in the table below:

Polyvinyl alcohol (PVOH) blends >98% >98% 85-96% 85-96% hydrolysis,hydrolysis, hydrolysis, hydrolysis, high low high high Solubilityviscosity viscosity viscosity viscosity Temperature PVOH PVOH PVOH PVOH° C. 75% 25% 70 60% 40% 30 80% 20% 40

Melt strength may be improved by increasing the ratio of highermolecular weight to lower molecular weight polyvinyl alcohols in ablend.

For example, a blend may comprise a low viscosity grade having amolecular weight in the range 13,000 to 27,000 and a degree ofpolymerization of 300-600 and a medium-high viscosity grade having amolecular weight in the range 107,000 to 120,000 and a degree ofpolymerization of 2,400 to 2,600.

In embodiments, the polyvinyl alcohol consists of a blend of two or morepolyvinyl alcohol polymers each having a degree of hydrolysis of 93% to98%, preferably one with a high molecular weight and at least one lowmolecular weight polyvinyl alcohol. In a preferred embodiment, thepolymer comprises 80% high molecular weight polyvinyl alcohol and 20%low molecular weight polyvinyl alcohol. The ratio of high to lowmolecular weight molecular polyvinyl alcohol may be about 2:1 to about10:1, alternatively about 3:1 to 7:1, alternatively about 6:1 to 4:1,alternatively about 5:1.

The high molecular weight polymer may have a molecular weight of 60,000to 120,000.

The lower molecular weight polymer may have a molecular weight of 5,000to 30,000.

The blends of different molecular weight polymers employed are selectedin accordance with the physical properties required in the finishedproduct. This may require different molecular weight materials beingused. Use of more than two different molecular weight polymers may beadvantageous. The use of a single molecular weight polymer is notprecluded.

Use of a blend may allow control of the viscosity of the polymer.Selection of a stabilizer in accordance with the present inventionallows use of blends of a desired viscosity without a loss of otherproperties. Alternatively, use of a blend may permit use of polyvinylalcohol with one or more stabilizers while maintaining viscosity orother properties to permit manufacture of pellets or films. Theprocessing aid is preferably water. Alternatively, the processing aidmay comprise a mixture of water and one or more hydroxyl compound with aboiling point less than the boiling point or melting point of theplasticizer. Use of water is preferred for cost and environmentalreasons.

When a mixture of plasticizers is employed, a binary mixture may bepreferred.

The plasticizer may be selected from the group consisting of:

(a) sugar alcohols selected from the group consisting of: diglycerol,triglycerol, fructose, ribose, xylose, D-mannitol, triacetin, andmixtures thereof; polyols selected from the group consisting of:pentaerythritol, dipentaerythritol, and mixtures thereof;

(b) diols selected from the group consisting of: methyl pentanediol,1,2-propanediol, 1,4-butanediol, 2-hydroxy-1,3-propanediol,3-methyl-1,3-butanediol, 3,3-dimethyl-1,2-butanediol, and mixturesthereof;

(c) glycols selected from the group consisting of: polyethylene glycol300, polyethylene glycol 400, alkoxylated polyethylene glycol, andmixtures thereof;

(d) caprolactam, tricyclic trimethylolpropane formal, rosin esters,euricamide, and mixtures thereof.

In a first embodiment, the following plasticizers may be used incombination:

dipentaerythritol, methyl pentanediol, triacetin,2-hydroxy-1,3-propanediol, 3,3-dimethyl-1,2-butanediol, tricyclictrimethylolpropane formal, D-mannitol, triglycerol, and xylose.

Preferably a binary composition of the plasticizers of the firstembodiment is employed.

In a second embodiment, the following plasticizers are used alone or incombination with each other or in combination with one or moreplasticizers of the first embodiment: caprolactam, alkoxylatedpolyethylene glycol.

An amount of 2-15 wt % of plasticizer may be used, wherein the totalamount of plasticizer in the formulation is from about 15 wt % to about30 wt %.

In a second embodiment, the following plasticizers are used alone or incombination with each other or with one or more plasticizers of thefirst embodiment: caprolactam, alkoxylated polyethylene glycol.

A suitable grade of dipentaerythritol is Di-Penta-93 (manufactured byPerstorp Corp.).

A suitable grade of caprolactam is Caprolactam 3031(manufactured byIngevity).

A suitable grade of alkoxylated polyethylene glycol is Alkoxylate 4528or Alkoxylate 3380 (manufactured by Perstorp Corp.).

In exemplary embodiments thin films of melt processed polyvinyl alcoholformulations of this invention remain clear and do not show a whiteningeffect after exposure to humidity. Use of an appropriate lubricant mayprovide films with improved visual appearance. An exemplary lubricant iseuricamide. In less advantageous formulations formation of an opaque,hazy or white film after a 24-hour humidity test may indicate that phaseseparation has occurred.

Preferred solid plasticizers or reactive stabilizers may also bevolatile under processing conditions at the processing temperature.Preferred plasticizers or reactive stabilizers have a melting point inthe range of about 150° C. to about 300° C., typically about 150° C. toabout 275° C.

A solution of the plasticized or stabilizer in water may be injectedthrough a single secondary inlet.

A solid plasticized or stabilizer may be fed independently or togetherwith one or more of the polymeric materials of the formulation.

Polymers containing the stabilizers and/or plasticizers of thisinvention provide films which may exhibit complete clarity after a24-hour humidity test.

One or more processing aids may be employed. Euricamide may be used as aslip additive. An amount of 0-5 wt % may be used.

Further additives may be used, including antioxidants, lubricants, dyesand pigments.

The water content of polymers of this invention may be in the range ofabout 0.1 wt % to about 5 wt %, for example, about 1 wt % to about 4 wt%.

The processing temperature may have a maximum of about 260° C.,dependent on dwell time in the higher temperature zones of the extruder.

In an exemplary embodiment, the mixing reactor comprises a twin screwextruder. Alternatively, the mixing reactor may comprise a batch reactorfor smaller scale processes. The mixing reactor should have a highinternal surface area to allow efficient heat dissipation.

The extruder reactor chamber may be composed of 5-20 heated regions,typically 10-15, more preferably about 12 regions. The temperatureprofile may rise from ambient

temperature at the first region to 200° C. adjacent the outlet. Thereaction zone may have a temperature of up to 260° C.

The location of the reaction zone may be controlled by selection andadjustment of one or more of: the screw configuration, the formulation,the temperature profile, rotational speed of the one or more screws(depending on reactor type) and the rate of feed of the reaction mixtureinto the apparatus. The location of the reaction zone may be determinedby the temperature as measured by one or more thermocouples or othertemperature sensors located arranged along the length of the chamber. Ina preferred embodiment, the reaction zone is controlled so that it islocated prior to the secondary outlet, upstream of the primary outlet.The location of the reaction zone may be adjusted so that the reactionis complete prior to venting.

Energy provided by the application of shear forces and control of thetemperature of the polymer mixture allows control of the chemicalenergetics of the exothermic reaction between the processing aid and thehydrogen bonded crystalline domains of the polymer followingcommencement of the exothermic reaction. Failure to affect adequatetemperature control may lead to decompositionand even carbonisation ofthe polymer mixture.

In a preferred embodiment, the configuration of the screws, typicallyco-rotating closely intermeshing twin screws of a twin screw extruder,may be as follows.

A conveying section may be provided at the throat or inlet of theextruder. The feed rate should be regulated so that the throat is notoverfed. The conveying section is followed by an intensive mixing zone,followed in turn by a conveying section which forms the reaction zone.In the reaction zone, the reaction goes essentially to completion. Thisis followed by an intensive mixing section in which the reaction isdriven fully to completion. Following the intensive mixing section,there is a low pressure zone where venting is allowed. A compressionzone then feeds the mixture to a dye, pump or simple screw extruder.

The temperatures which may be used for a blown film formulation are asfollows:

Zone 0 1 2 3 4 5 6 7 8 9 10 11 12 Temp. (° C.) x 20 22 75 155 195 200200 200 200 200 200 200

The temperatures which may be used for an extrusion coating formulationare as follows:

Zone 0 1 2 3 4 5 6 7 8 9 10 11 Die Temp. (° C.) x 10 10 75 200 235 250250 250 230 230 225 225

The temperature of the reaction mixture may not be the same as the setpoint values because of mechanical heating stemming from the mixingprocess, shear heating effects and the reaction exotherm and the poorheat transfer to coolant in a steel reaction vessel. Those skilled inthe art are capable of judging the appropriate processing conditions.

An intensive mixing region may be provided downstream of the reactionzone to ensure completion of the reaction of the processing aid andpolymer. In a preferred embodiment, the intensive mixing region maycomprise a paddle mixer located between the reaction zone and thesecondary outlet. The mixing region may be a kneading region comprisingpairs of inter-engaging rotor blades or paddles.

Preferred mixing reactors are self-cleaning in use. Co-rotatingintermeshing screws as used in twin screw extruders may be employed. Thestated operating conditions may be employed using appropriate start-upand shut-down procedures.

For start-up, a completely empty and clean extruder barrel may be used.The water or processing aid feed is started, followed by the polymerpowder and plasticizer either simultaneously or successively. Theinitial feed rate and screw rotation are lower than the steady statespeeds. In the case where the die is connected directly to the twinscrew extruder, once a coherent strand is produced the feed rate andscrew speeds are raised to the steady state conditions. The fitting of adry face cutter or strand pelletiser is carried out in the usual wayknown to those skilled in the art. When a single screw is employed, thismust be empty and connected to the twin screw extruder prior tostart-up.

Pre-flushing with standard flushing agents such as low-densitypolyethylene, high density polyethylene or polypropylene, whether filledor unfilled, is neither necessary nor desirable when using the processof the present invention. When the die is attached to the single screwextruder pelletiser filling is as described above.

In order to provide a clean extruder for a subsequent start-up, theshut-down procedure may involve stripping all feeds and reducing allscrew speeds and continued running until as much material as possiblehas been delivered. Where the twin screw extruder is coupled to a singlescrew extruder or other form of melt pump, the twin screw may bedecoupled from the single screw and the die may also be uncoupled. Thedie is placed in a heated oven at 300-450° C. to burn off any remainingpolymer or soaked in hot water until the polymer dissolves or becomesswollen so that it can be readily removed mechanically. The temperatureof the decoupled twin screw extruder may be then lowered to a uniform100-110° C. with rotation of the screws so that the residual polymer isejected as crumb-like material until the barrels are empty. The barrelmay be then polished by feeding some of the dried powdered polymer.After the polishing stage, the final residual material is ejected.

In the case of a single screw extruder, the optimal processingtemperature of about 200° C. is maintained. If a closed barrel extruderis used, the screws may be decoupled and removed slowly without cooling.The polymer is pulled from the screws as it is withdrawn from thebarrel. This affords a clean screw. If a clam-shell single screwextruder is used, the casing may be opened and the polymer removedquickly while hot before removing the heated screws. The barrel may becleaned with a wire brush during cooling. Flushing the extruder barrelswith a purged material is neither necessary nor effective.

The secondary outlet may be a vent permitting volatile processing aids,for example steam, to be completely or partially removed from thepolymer mixture.

When water is the processing aid, the water content of the plasticisedpolymer may be less than 5 wt %, preferably less than 2 wt %, morepreferably not more than 0.5 wt %.

In an alternative embodiment, an inlet for the processing aid is locatedupstream of the inlet for the plasticizer. This allows the polymer tomix with the processing aid before the plasticizer is introduced.Without wishing to be bound by theory it is believed that the moleculesof a plasticizer such as neopentyl glycol may be slow to break into thecrystalline domains of the polyvinyl alcohol. Energy provided by theapplication of shear forces to and control of the temperature of thepolymer mixture allows control of the chemical energetics of theexothermic reaction between the processing aid and the hydrogen bondedcrystalline domains of the polymer following commencement of theexothermic reaction. Failure to effect adequate control may lead todegradation and even carbonisation of the polymer.

The rate of rotation of the twin screws may be regulated to control thespecific energy per unit length of the reactive mixing chamber so thatthe screws serve as energy input devices.

In a typical embodiment, the chamber of the mixing reactor may be30%-70% filled with the polymer mixture with the remaining volume beingempty or serving as a lower pressure zone to facilitatedevolatilisation. Consequently, the rate of output of polymer from theprimary outlet may not be consistent and may be pulsed. A compactionzone may be employed to provide a continuous output.

In a preferred embodiment, the mixing reactor is a twin screw extruderhaving a preferred length to diameter ratio in the range 25:1 to 50:1,preferably about 25:1 to 45:1, more preferably about 40:1. For example atypical mixer reactor may have two 95 mm diameter screws with a lengthof 4.8 metres.

The primary outlet may comprise a die, for example a multistrand die.

Alternatively, in an advantageous embodiment of the invention, a pumpmay be provided downstream of the primary outlet. The pump may comprisea single screw extruder unit. The configuration and speed of rotation ofthe single screw may be selected so that the pump is full of plasticisedpolymer during use. In this way the screw serves as a variable pumpwhich is controllable to provide a constant flow of polymer to a dielocated downstream of the reactive mixer.

Application of shear by mixing a temperature-controlled mixture ofpolyvinyl alcohol and water, or other processing aid, gives rise to anexothermic reaction which when properly controlled serves to reduce ordestroy crystallinity of the high degree of hydrolysis polyvinylalcohol. Without wishing to be bound by theory, it is believed thatlattice energy of the crystalline polyvinyl alcohol is released by theintroduction of hydrogen bonding due to incorporation of water, or otherprocessing aid, into the polymer mixture.

The onset of the exothermic reaction may be controlled by selection ofthe temperature profile and shear rate applied in the twin screws. Theextent of the exothermic reaction may be controlled by the rate of heatremoval from the mixer, by the composition and feed rate of the reactionmixture, and by regulation of the shear energy input and location of thereaction zone. The location of the reaction zone at which the exothermicreaction occurs may be controlled by appropriate control of thetemperature profile and rate of rotation.

The boiling point of the processing aid is preferably selected so thatit is less than the temperature of the reaction and mixing zones,permitting venting of excess processing aid from the polymer mixture.

The mean residence time in the mixer may be about 2-10 minutes,preferably about 5 minutes. The residence time in the reactor ispreferably sufficient to allow completion of the reaction so that aviscoelastic melt is obtained with a minimum amount of unreactedpolyvinyl alcohol.

A cooling chamber may be located downstream of the die. This maycomprise a system of moving rollers located in a controlled atmospherearranged so that the polymer strands emerging from the die aremaintained under appropriate tension as they cool and solidify prior topelletization.

The percentages referred to in this specification may be selected fromany of the ranges quoted to total 100%. Percentages or other quantitiesused in the specification are by weight unless indicated otherwise.

According to a second aspect of the present invention a homopolymericpolyvinyl alcohol polymer composition comprises: homopolymeric polyvinylalcohol having a degree of hydrolysis of 93% or higher,

a plasticizer and a reactive stabilizer;

wherein the plasticizer is selected from the group consisting of: sugaralcohols, diols, triols, polyols and mixtures thereof;

wherein the reactive stabilizer is selected from the group consistingof:

sodium stearate, potassium oleate, sodium benzoate, calcium stearate,stearic acid, dimethyl propionic acid, and mixtures thereof;

wherein the polymer is a viscoelastic thermoplastic material.

According to a third aspect of the present invention a method ofreducing degradation of homopolymeric polyvinyl alcohol duringprocessing includes the steps of the method of the first aspect of thisinvention.

The polyvinyl alcohol composition of this invention provides manyadvantages in relation to previously used compositions. Exemplarycompositions are extrudable and can be used for making pellets, filmsand fibres.

The invention is further described by means of example but not in anylimitative sense.

EXAMPLE Example 1 Use of Sodium Benzoate

Sodium benzoate was used as a reactive stabilizer with variousplasticizers and combinations of plasticizers as listed below:

trimethylolethane (TME) and trimethylolpropane (TMP) 1:2 ratio

trimethylolethane (TME) and trimethylolpropane (TMP) 1:1 ratio

trimethylolethane (TME) and trimethylolpropane (TMP) 2:1 ratio

neopentyl glycol (NPG) and trimethylolpropane (TMP) 1:1 ratio

neopentyl glycol (NPG) and trimethylolpropane (TMP) 2:1 ratio

neopentyl glycol (NPG) and trimethylolpropane (TMP) 3:1 ratio

tripentaerytheritol (TPE) and trimethylolpropane (TMP) 1:4 ratio

diethylpentanediol dineopentanoate (DEPD) and trimethylolpropane (TMP)1:3 ratio

diethylpentanediol dineopentanoate (DEPD) and trimethylolpropane (TMP)1:2 ratio

diethylpentanediol dineopentanoate (DEPD)and trimethylolpropane (TMP)1:1 ratio

Sodium benzoate was used as the reactive stabilizer and the plasticizercombinations and ratios were changed.

The results show that when using sodium benzoate as a reactivestabilizer and changing the plasticizer combinations and plasticizerratios the degradation of the polymer can be controlled. For example,with the NPG:TMP plasticizer combination increasing the ratio of NPG tothe TMP results in a larger increase in the degradation of the polymer.When using the same plasticizer ratio of 3:1 but with differentplasticizers specifically, pentaerythritol, TME and NPG, it was foundthat the TME:TMP combination was superior and had the least degradation.From the degradation data the preferred plasticizer combinations areTME:TMP and DEPD:TMP.

Crystallinity of the polymer is important as it can affect the secondaryand tertiary processes needed to create a final product. For example, ifthe film is too crystalline then the ability to process the film intoproduct is hindered as it is too brittle. Crystallinity of the polymerin the form of a pellet, film or conditioned film (that is film that hasbeen exposed to humidity) has been compared. The results are shownbelow.

Effect on crystallinity by use of reactive stabilizers in pellet, filmand conditioned film

The addition of a reactive stabilizer in accordance with this inventionmay reduce the crystallinity of the film. In general, the crystallinityvalues of the pellets were slightly higher or similar to that of thefilms. However, there was no significant change in the crystallinitybefore and after humidity treatment.

When an alternative plasticizer was used in the formulation, the Tgvalue could be observed by DSC analysis. The Tg values obtained were inthe region of 44-65° C. The melting peak (Tm) for all of the reactivestabilizer and plasticizer combinations were within the region of200-215° C. Therefore, from a thermal perspective the combinations ofplasticizers and reactive stabilizers are shown to be suitable forprocessing. However, a preferred combination is sodium stearate with theplasticizer combination TMP and pentaerythritol.

The results are shown in the following Table:

Plasticiser Reactive Stabiliser Tg ° C. Tm ° C. (peak) Polyol 3990/Pentabis-MPA 57 208 TMP/Penta sodium benzoate 47 211 TMP/Penta potassiumoleate 47 210 TMP/Penta sodium stearate 44 209 D-Mannitol sodiumstearate 49 211 MPD/Di penta sodium benzoate 59 213

The phase separation of a film which does not contain a reactivestabilizer was observed after 24 hours. The film became milky. Usingsodium benzoate as the stabilizer in combination with variousplasticizers and combinations of plasticizers resulted in a clear filmafter conditioning.

Torque analysis of the polyvinyl alcohol homopolymers stabilized withvarious reactive stabilizers was carried out in a batch mixer.Processing was performed with total batches of 40 g at 190° C. and 60rpm for 8 min. All stabilizers reduced the torque. A combination ofcalcium stearate with stearic acid gave the biggest reduction. Theresults are shown below:

Max Torque Torque Levelled Reactive stabiliser Temp (Nm) Value (Nm) Nostabiliser 190° C. 17.7 9.6 calcium stearate 190° C. 13.7 8.2 stearicacid 190° C. 11.4 8.2 calcium stearate and stearic acid 190° C. 9.3 7.7Sodium stearate 190° C. 10.7 7.7 Sodium benzoate 190° C. 14.9 9.2potassium oleate 190° C. 12.7 8.0

Reactive stabiliser Tm ° C. Crystallization % No stabiliser 206 45calcium stearate 202 59 stearic acid 205 58 calcium stearate 203 59 andstearic acid sodium stearate 206 52 sodium benzoate 206 49 potassiumoleate 204 51

1. A method for manufacture of plasticized homopolymeric polyvinylalcohol, the method comprising the steps of: introducing into a mixingreactor a polyvinyl alcohol polymer comprising polyvinyl alcohol or ablend thereof having a degree of hydrolysis in the range of 93 wt % to98 wt %; wherein the mixing reactor comprises a blending chamber havinga primary inlet, a primary outlet and at least two inter-engagingcomponents extending between the primary inlet and primary outlet, thecomponents being arranged to apply a shearing force to the polymer whilethe polymer is conveyed by the components from the inlet through areaction zone to the outlet; one or more secondary inlets locateddownstream from the primary inlet for introducing reactants comprising aprocessing aid, a plasticizer and a reactive stabilizer to the chamberto form a reaction mixture; wherein the plasticizer is selected from thegroup consisting of: sugar alcohols, diols, triols, polyols and mixturesthereof; wherein the reactive stabilizer is selected from the groupconsisting of: sodium stearate, potassium oleate, sodium benzoate,calcium stearate, stearic acid, dimethyl propionic acid, and mixturesthereof; wherein the blending chamber comprises a plurality of heatedregions arranged so that the mixture is subjected to a temperatureprofile whereby the temperature increases from the inlet to the outlet;a secondary outlet located between the reaction zone and primary outletarranged to allow removal of processing aid from the chamber; reactingthe processing agent, plasticizer and polymer in the reaction zone toform plasticized polymer; and allowing the plasticized polymer to passfrom the primary outlet.
 2. A method as claimed in claim 1, wherein thereactive stabilizer is sodium benzoate.
 3. A method as claimed in claim1, wherein the amount of the reactive stabilizer is from 0.2 wt % to 5wt %.
 4. A method as claimed in claim 3, wherein the amount of thereactive stabilizer is from 0.5 wt % to 3 wt %.
 5. A method as claimedin claim 4, wherein the amount of the reactive stabilizer is from 0.5 wt% to 2 wt %.
 6. A method as claimed in claim 5, wherein the amount ofreactive stabilizer is from 0.5 wt % to 1.5 wt %, preferably 1 wt %. 7.A method as claimed in claim 1, wherein the polyvinyl alcohol polymercomprises polyvinyl alcohol or a blend thereof wherein the polyvinylalcohol has a degree of hydrolysis of 93wt % to less than 98wt %.
 8. Amethod as claimed in claim 7, wherein the degree of hydrolysis is 93wt %to 97wt %.
 9. A method as claimed in claim 8, wherein the degree ofhydrolysis is 93wt % to 95wt %.
 10. A method as claimed in claim 1,wherein the polyvinyl alcohol polymer is a blend of two or morepolyvinyl alcohol polymers with a relatively high molecular weight and arelatively low molecular weight respectively.
 11. A method as claimed inclaim 10, wherein the blend comprises a low viscosity grade having amolecular weight in the range 13,000 to 27,000 and a degree ofpolymerization of 300-600 and a medium/high viscosity grade having amolecular weight in the range 107,000 to 120,000 and a degree ofpolymerization of 2,400 to 2,600.
 12. A method as claimed in claim 1,wherein the plasticizer is selected from the group consisting of: (a)sugar alcohols selected from the group consisting of: diglycerol,triglycerol, fructose, ribose, xylose, D-mannitol, triacetin, andmixtures thereof; polyols selected from the group consisting of:pentaerythritol, dipentaerythritol, and mixtures thereof; (b) diolsselected from the group consisting of: methyl pentanediol,1,2-propanediol, 1,4-butanediol, 2-hydroxy-1,3-propanediol,3-methyl-1,3-butanediol, 3,3-dimethyl-1,2-butanediol, and mixturesthereof; (c) glycols selected from the group consisting of: polyethyleneglycol 300, polyethylene glycol 400, alkoxylated polyethylene glycol,and mixtures thereof; (d) caprolactam, tricyclic trimethylolpropaneformal, rosin esters, euricamide, and mixtures thereof.
 13. A method asclaimed in claim 1, wherein two or more of the following plasticizersare used in combination: dipentaerythritol, methyl pentanediol,triacetin, 2-hydroxy-1,3-propanediol, 3,3-dimethyl-1,2-butanediol,tricyclic trimethylolpropane formal, D-mannitol, triglycerol, andxylose.
 14. A method as claimed in claim 1, wherein the followingplasticizers are used alone or in combination with each other:caprolactam, alkoxylated polyethylene glycol.
 15. A plasticizedpolyvinyl alcohol polymer manufactured by the method of claim 1.